Use platform ID to choose the right reader for AVHRR GAC data

[mraspaud]

Using platform ID, we can skip reading the file at the init stage altogether.

• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[codecov]

Codecov Report

Merging #674 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
- Coverage   78.66%   78.66%   -0.01%     
==========================================
  Files         138      138              
  Lines       20356    20357       +1     
==========================================
  Hits        16013    16013              
- Misses       4343     4344       +1

Impacted Files	Coverage Δ
satpy/readers/avhrr_l1b_gaclac.py	8.33% <0%> (-0.18%)	⬇️
satpy/writers/geotiff.py	42.01% <0%> (ø)	⬆️

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[coveralls]

Coverage decreased (-0.02%) to 78.641% when pulling f4f5cdb on mraspaud:fix-gac-reader into 40a0dcd on pytroll:master.